Natural gas, when cooled to approximately −260° F., changes phase from a gas to a liquid, thus “Liquefied Natural Gas” or “LNG.” In this phase change process, the volume required to hold a specific quantity of natural gas is reduced by approximately 600 times, thus making it possible to transport significant, and economic quantities of natural gas over great distances from source to market.
LNG is a boiling cryogen that is usually stored at atmospheric temperature and pressure equilibrium conditions. Unlike other gaseous fuels such as propane and butane, which can be stored as a liquid at atmospheric temperatures by allowing the liquid and the gas in the tank to reach a stable equilibrium vapor pressure for any given atmospheric temperature, LNG (the principal component of which is methane) cannot be maintained as a liquid under pressure at atmospheric temperatures due to its low critical point pressure (673+ psia for methane), critical point temperature (−115.8° F. for methane), and very high vapor pressures. Accordingly, LNG is stored and is transported in heavily insulated tanks.
Although the amount of heat that reaches the LNG is significantly reduced by the tank insulation, the heat inflow to the LNG cannot be entirely eliminated. Consequently, a quantity of cold natural gas vapor (referred to as “boil off vapor” or “boil off”) is constantly being generated and must be removed from the tank and must be either disposed of or re-liquefied in order to prevent an overpressure condition of the LNG tank. Specifically, the resulting boil off is either: (1) vented to the atmosphere (which venting is limited, by regulation, as an emergency/extraordinary procedure because natural gas is flammable and is a significant greenhouse gas); (2) heated, pressurized, and sent to a gas distribution system (in the case of land-based LNG tanks); (3) re-liquefied and returned to the tank as LNG; (4) flared as waste gas; (5) burned in propulsion machinery as fuel (in the case of liquefied natural gas carriers, or “LNGCs”); or (6) contained in the LNG tank for a finite period of time by allowing the vapor space of the tank to increase in pressure as the LNG continues to boil. This latter option can only be sustained for a relatively short period of time, typically limited to days (generally less than a month).
Historically, LNG has been utilized to effect the transportation of natural gas from sources in remote regions of the world to end users in population centers where demand for energy, particularly natural gas, is continually increasing. LNG has also been utilized for the purpose of efficiently storing natural gas during periods of low natural gas demand for later use during periods of high natural gas demand, i.e., so called “peak shaving” operations.
Recently, LNG is increasingly being utilized as a feedstock for generating and industrial facilities and as a transportation fuel for both land and marine vehicles. Natural gas is an attractive transportation fuel from the perspectives of long term availability, reduced emissions, and cost advantage over traditional distillate fuels. However, to achieve an equivalent energy level, the size of the space needed to house the required quantity of LNG is substantially greater than the size of the space needed to house the required quantity of a light distillate fuel, such as diesel fuel.
The increased demand for and use of LNG is creating a need for additional waterborne strategies for transportation of LNG to end-user distribution facilities. The marine transportation and distribution of LNG, whether in inland rivers and waterways or on open ocean coastal routes, is often most efficiently and economically accomplished by systems that utilize tugboats and barges.
In the case of the only LNG barge to be built (see Donald W. Oakley, World's First Commercial LNG Barge, OCEAN INDUSTRY, November 1973, at 29-32), the LNG boil off was allowed to accumulate in the LNG tank by allowing the pressure in the tank to increase over time. The LNG tanks and the insulation system were designed to contain the boil off for a period of 45 days before the LNG tank relief valves would open due to overpressure, thus releasing the natural gas to the atmosphere.
A significant problem with this approach is that the LNG itself will rise in temperature to reach the equilibrium temperature that corresponds to the pressure of the LNG tank. Specifically, as the LNG tank pressure rises, the LNG temperature will also rise. If this warm LNG is then pumped into an LNG storage tank that is at a lower/normal pressure (i.e., a pressure that is slightly above atmospheric pressure, e.g., approximately +100 millibars), the warm LNG will rapidly vaporize and will release large volumes of cold natural gas as the LNG is cooled by evaporative processes until the LNG again reaches an equilibrium temperature that corresponds to the new tank pressure. This is unacceptable, since an LNG receiving terminal will be unable to dispose of the excess natural gas and tank overpressure is likely, with subsequent release of natural gas to the atmosphere. Even a slightly warmer LNG can be problematic due to the phenomenon of “roll-over” within the storage tank resulting in rapid and uncontrolled LNG vaporization.
There is also an increased safety risk associated with LNG at equilibrium conditions that are above atmospheric pressure should the LNG be accidentally released. At higher pressure equilibrium conditions, the LNG will vaporize at an increased rate, thereby significantly increasing thermal radiation risks should the vapor cloud ignite prior to dispersing.
Self-propelled LNGCs use the boil off as propulsion fuel in the ship's engines and are, therefore, able to maintain proper LNG tank pressure and LNG temperature. In the case of a barge, however, this approach is not an option because a barge does not have propulsion engines.
The LNG barge referred to above solved this problem of the increasing LNG temperature with time by cooling the LNG in a controlled fashion during the discharge operation, prior to the LNG being pumped into land-based tanks. This process was described by Mr. Oakley in the November 1973 OCEAN INDUSTRY article and will not be repeated herein. Such cooling process, depending on the length of time that the LNG is aboard the barge and other factors, can result in discharge delays and considerable additional expense. It also significantly complicates the discharging operation. Finally, the added LNG cooling equipment that is required is costly to purchase and is expensive to maintain.
U.S. Pat. No. 7,047,899 to Laurilehto et al. teaches the concept of using electric generator sets that are fitted to a barge and use natural gas as fuel, thereby allowing cargo tank boil off to be consumed in the engines, thereby allowing for control of the pressure of the cargo tanks. Electrical propulsion power for a tugboat is transferred to the tugboat from the barge by electrical cables. U.S. Patent Application Publication No. 2006/0053806 to Van Tassel also teaches several approaches for effectively managing LNG cargo tank boil off and, therefore, LNG cargo tank pressure.
An article entitled LNG-Power Is the Time Now? published in the February 2011 issue of MARINE NEWS, teaches the concept of using an LNG fuel barge combined with a typical inland towboat to provide natural gas fuel to the towboat as there is generally insufficient space on the towboat to house a sufficient quantity of LNG fuel. This article describes transferring LNG to the towboat in liquid, cryogenic form and processing and re-gasifying the liquid gas on the towboat, so that the engines of the towboat can make use of the gas as fuel. Such transfer of cryogenic gas is extremely hazardous owing to both the cryogenic temperatures involved and the increased likelihood and consequences of leakage.
FIG. 5 of U.S. Pat. No. 2,795,937 to Sattler et al. (“Sattler”) discloses the transfer of the boil off gas from cargo tanks on a barge to a tugboat that tows (in this case, pulls) the barge. In FIG. 5, Sattler discloses that the boil off gas is to be transferred from the barge to the tugboat through a flexible conduit or pipe. The boil off gas is then to be used as fuel in the tugboat's propulsion power plant, in this case a steam boiler, in much the same manner as in a self-propelled ship (e.g., a LNGC). By consuming the boil off in the tugboat's propulsion system, the LNG cargo tank pressure can, therefore, be maintained at near atmospheric pressure.
An examination of Sattler reveals, however, that Sattler fails to recognize the many significant technical, operational, and regulatory problems that would prevent the embodiment shown in FIG. 5 from ever becoming operative. (In fact, to the best of the inventor's extensive knowledge of this field, such an embodiment has never been reduced to practice.) The most significant of these problems is the high likelihood that the flexible conduit or pipe would be damaged or severed by the unrestricted relative motion and resulting forces between the barge and tugboat, which is further aggravated by the typical distances (often in excess of 600 feet) that a barge is towed behind a tugboat. As a consequence, natural gas would be released to the atmosphere, creating a potentially hazardous situation due to the release of significant quantities of natural gas. At a minimum, this will contribute adversely to greenhouse gas emissions. Additional problems that Sattler fails to recognize include: (1) the flexible conduit or pipe would have to be able to accommodate motion in all degrees of freedom, as a tugboat and barge have complete freedom of motion relative to each other; (2) natural gas would be released to the atmosphere when connecting and disconnecting the flexible conduit or pipe; (3) you would have to find a way of purging the flexible conduit or pipe with inert gas prior to connecting or disconnecting the flexible conduit or pipe; (4) there does not appear to be any provision for emergency breakaway and disconnection of the flexible conduit or pipe should the tugboat need to separate from the barge or should the towline part, which is not uncommon; (5) there does not appear to be any provision for secondary containment of natural gas should the flexible conduit or pipe fail or leak; (6) there does not appear to be any provision to detect the leakage of natural gas should the flexible conduit or pipe develop a leak, or to detect leakage at the connections of the flexible conduit or pipe; and (7) there is no automatic shutdown of the natural gas transfer from the barge to the tugboat upon failure or leakage of the flexible conduit or pipe or its connections.